1. Field of the Invention
The present invention relates to a manufacturing method of a thin film device substrate which comprises the step of transferring a thin film device formed on one substrate onto another substrate.
2. Description of the Related Art
For the switching element in the pixel section of the liquid crystal display unit, the thin film transistor (referred to as the “TFT” hereinafter) has been being widely used. In recent years, with the object of achieving the liquid crystal display unit of higher precision as well as producing the system on glass (referred to as the “SOG” hereinafter) successfully, there have been growing demands for higher operation speeds. The SOG herein is a technique to integrate, along with the TFT for driving the liquid crystal, the TFTs in adjacent circuits such as the driver circuit, the memory and the CPU (Central Processing Unit) onto one glass substrate.
(First Conventional Technique)
As the typical method of forming a polycrystalline silicon film on a glass substrate, there can be given the laser annealing method. In the ordinary laser annealing method, after a silicon oxide film is formed as a buffer layer on a glass substrate and an amorphous silicon film is formed on this silicon oxide film, the amorphous silicon film made molten by laser irradiation is recrystallized to form a polycrystalline silicon film.
Meanwhile, the method of increasing the grain size of the crystal is under investigation, since the operation speed of the polycrystalline silicon TFT rises with increasing grain size thereof. For instance, as an attempt to make the grain size of the crystals larger through the use of annealing, there is a method of forming a porous silicon oxide film with a thermal conductivity lower than the ones of conventionally employed silicon oxide films as an underlying layer of the amorphous silicon film (Japanese Patent Application Laid-open No. 91604/2000). When the temperature of amorphous silicon film increases owing to the laser annealing, the thermal diffusion from the amorphous silicon film to the substrate takes place. The presence of the porous silicon oxide film which is set to underlie the amorphous silicon film reduces the thermal diffusion rate so that the melting time period for the amorphous silicon film hereat becomes longer than that in the ordinary laser annealing methods. As a result, a polycrystalline silicon film whose particle size is larger than the usual ones may be obtained.
(Second Conventional Technique)
Next, the substrate materials are described. For example, when the liquid crystal display device making use of the TFT is fabricated, a glass substrate is not necessarily the best material. The glass substrate is disadvantageously heavy, fragile and inflexible. As against this, as the substrate which is lighter, less fragile and more flexible, there can be given the plastic substrate. However, the TFT formed on the plastic substrate has the disadvantage of poorer performance than that of the TFT formed on the glass substrate, since the processing temperature for the TFT formed on the plastic substrate is limited to 150° C. or so. Accordingly, there have been brought forward several methods (referred to as the “substrate peeling-off methods” hereinafter) wherein, after a plastic substrate is bonded onto a TFT formed on a glass substrate, the glass substrate is peeled off therefrom and thereby the TFT is transferred onto the plastic substrate. They are further described below.
In a method disclosed in Patent Literature 2, by forming numerous trenches in a glass substrate and then forming a silicon oxide film on the glass substrate by means of spin-coat, formation of gaps in the trenches is made, and following the TFT formation, an etching solution is permeated into these gaps, and thereby the glass substrate is peeled off. In a method disclosed in Patent Literature 3, an isolation layer made of amorphous silicon or the like is laid on a glass substrate that can transmit a laser light, and thereon a TFT is formed. After another substrate is adhered onto this TFT, backside irradiation with the laser light is applied thereto, whereby the bonding force of the isolation layer is weakened so as to separate the glass substrate therefrom. In a method disclosed in Patent Literature 4, a glass substrate is characteristically removed by chemical polishing.
(First Problem)
As described in First Conventional Technique, a polycrystalline silicon film of large grain size can be attained by applying laser annealing onto an amorphous silicon film overlying a porous silicon oxide film. However, because the porous silicon oxide film hinders thermal diffusion, the TFT laid on the porous silicon oxide film tends to make abnormal operations when overheated. Furthermore, the porous silicon oxide film is fragile so that the overall mechanical strength of the whole TFT substrate becomes less than satisfactory.
(Second Problem)
For the substrate peeling-off methods described in Second Conventional Technique, the following problems are known. In the method of Japanese Patent Application Laid-open No. 288522/1996, the step of forming trenches in a glass substrate is comprised so that the glass substrate surface becomes rough, which lowers the pattern accuracy of the thin film device formed on the glass substrate. In the method of Japanese Patent Application Laid-open No. 26733/1999, because the peeling-off is made with a laser, a material such as a glass having a low transmittance for the laser light or a silicon impervious to laser light cannot be employed for the substrate. In the method of Japanese Patent Application Laid-open No. 212116/1999, the use of the polishing method lengthens its processing tome period.
Accordingly, an object of the present invention is to provide a method of manufacturing a thin film device substrate which can well prevent the thin film device formed by laser annealing from making, due to overheating, abnormal operations, and can provide the substrate peeling-off method wherein no trench fabrication is required to be applied onto the substrate surface, and a material which is impervious to light can be used, and besides the substrate can be peeled off in a short period of time.